How do martensitic twin boundaries move?

The martensitic phase transformation from bcc to hcp underlies a number of curious effects, including shape-memory and superelasticity. The distinctive feature is the microscopic reversibility of the transition: at the atomic level, the position of each atom is uniquely related between phases. Moreover, this one-to-one relationship holds not only for the perfect crystal transformation mechanism, but also for the topological defects (such as twin boundaries) which are created in the transformation. Furthermore, for shape memory effect the microscopic relationship must be preserved by the deformation mechanism active in the material. In this paper, we examine the microscopic phenomena which allow these relationships to hold, and the consequences for shape-memory alloy design.